Implantable pulse generators (or IPGs) are well known in the prior art. Most demand pacemakers include sense amplifier circuitry for detecting intrinsic cardiac electrical activity so that the devices may be inhibited from generating unnecessary output stimulating pulses when a heart is functioning properly.
Dual-chamber cardiac pacemakers typically have separate sense amplifiers for atrial and ventricular sensing. The sense amplifiers detect the presence of intrinsic signals, that is P-waves occurring naturally in the atrium and R-waves occurring naturally in the ventricle. Upon detecting an intrinsic signal, sense amplifier circuitry generates a digital signal for output to other components which inhibit the delivery of a pacing pulse to the corresponding chamber.
It is desirable to accurately and reliably measure the response of the heart to an electrical stimulation pulse. Measuring such a response permits the determination of a patient's stimulation threshold, or the minimum energy a stimulating pulse must contain for a cardiac response to be evoked. Once a patient's stimulation threshold is determined, the energy content of stimulating pulses may be adjusted to avoid delivering pulses having unnecessarily high energy content. Minimizing the energy content of stimulating pulses is believed to have physiological benefits, and additionally reduces power consumption, a key concern in the context of battery-powered implantable devices.
Detection and measurement of the response of the heart to an electrical stimulating pulse may also be useful in controlling a pacemaker's pacing rate, for ascertaining the physiological effect of drugs or for diagnosing abnormal cardiac conditions.
Immediately following delivery of a pacing pulse to cardiac tissue, a residual post-pace polarization signal (or polarization signal) is generated by the charge induced in the tissue by delivery of a pacing pulse. If the pacing pulse causes an evoked response in the cardiac tissue, then an evoked response signal is superimposed atop the typically much larger amplitude polarization signal. As a result, conventional pacemakers or pacemaker-cardioverter-defibrillators (PCDs) either cannot differentiate, or have difficulty differentiating, between post-pacing pulse polarization signals and evoked response signals. This problem is further complicated and exacerbated by the fact that residual polarization signals typically have high amplitudes even when evoked response signals do occur. Consequently, it becomes difficult, if not impossible, to detect an evoked response signal using a conventional pacemaker or PCD sense amplifier employing linear frequency filtering techniques. As a result, most pacemakers cannot discern between polarization signals and evoked response signals.
Most pacemakers employ sensing and timing circuits that do not attempt to detect evoked response signals until the polarization signal-is no longer present or has subsided to some minimal amplitude level; only then is sensing considered reliable. In respect of capture detection, however, such sensing typically occurs a significant period of time after the evoked response signal has already occurred. As a result, most pacemakers cannot detect evoked response signals with any degree of confidence.
The generation and delivery of an electrical heart stimulating pulse gives rise to the storage of charge in body tissues. Such stimulation polarization artifacts, "after potentials," or polarization signals typically have much larger amplitudes than those corresponding to electrical signals arising from an intrinsic heartbeat or a stimulated response. Polarization signals may also interfere with the detection and analysis of a stimulated or evoked response to a pacing pulse. Thus, a need exists in the medical arts for determining reliably whether or not an evoked response signal has occurred in a pacing environment.
Polarization signals typically arise due to the tissue-electrode interface storing energy after a pacing stimulus has been delivered. There are typically two tissue-electrode interfaces in a pacing circuit: one for the tip electrode, and one for the ring (or canister) electrode. The stored energy dissipates after the pace event, creating the after-potential.
In respect of the impedance sensed by a pacemaker's internal circuitry, the total load of the pacing circuit comprises the impedance of the lead itself, the tissue-electrode interface impedances, and the impedance of the body tissue bulk. The impedances of the body tissue and the lead may be modeled as a simple series bulk resistance, leaving the tissue-electrode interfaces as the reactive energy absorbing/discharging elements of the total load. The tip electrode is the primary after-potential storage element in comparison to the case and ring electrodes. In a pacing circuit, a ring electrode typically stores more energy than does a case electrode due to differences in electrode areas.
Several methods have been proposed in the prior art for improving an implantable device's ability to detect and measure evoked responses. is For example, U.S. Pat. No. 5,172,690 to Nappholz et al., entitled "Automatic Stimulus Artifact Reduction for Accurate Analysis of the Heart's Stimulated Response," hereby incorporated by reference herein-its entirety, proposes a tri-phasic stimulation waveform consisting of precharge, stimulus, and postcharge segments. The duration of the precharge segment is varied until the amplitude of the stimulation artifact is small compared to the evoked response.
U.S. Pat. No. 5,431,693 to Schroeppel, entitled "Method of Verifying Capture of the Heart by a Pacemaker," hereby incorporated by reference herein its entirety, discloses a pacemaker that low-pass filters a sensed signal to remove noise and pass frequencies characteristic of the evoked cardiac signal. The filtered signal is processed to render a waveform signal representing the second derivative of the filtered signal. The second derivative filtered signal is further analyzed to detect minimum and maximum amplitude excursions during selected first and second time windows. The amplitude differences measured during the two time windows are compared to one another to determine whether capture has occurred.
U.S. Pat. No. 5,571,144 to Schroeppel, entitled "Method of Verifying Capture of the Heart by a Cardiac Stimulator," hereby incorporated by reference herein its entirety, discloses a pacer that
U.S. Pat. No. 4,114,627 to Lewyn et al., entitled "Cardiac Pacer System and Method with Capture Verification Signal," hereby incorporated by reference herein its entirety, discloses a pacer that delivers output stimulating pulses through an output coupling capacitor. During delivery of a stimulating pulse, the sense amplifier is uncoupled from the cardiac electrode. When the stimulating pulse terminates, the output coupling capacitor is coupled to ground through a discharge resistor, thereby discharging electrode polarization.
German Patent No. 4,444,144 to Hauptmann entitled "Pacemaker with Improved Sensing Circuit for Electrical Signals," hereby incorporated by reference herein in its entirety, discloses a pacemaker having a sensing circuit which records intracardiac heart signals. An adaptive non-linear noise filter transforms those signals. A matched filter correlates the transformed signals to a pulse pattern and creates an output indicative of heart pulse signals. The sensing circuit reduces faulty signal detection caused by noise filtering by permitting external noise to be distinguished noise associated with true heart signals.
As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and claims set forth below, many of the devices and methods disclosed in the above-noted patents may be modified advantageously by using the teachings of the present invention. What is needed is an implantable pulse generator that is capable of reliably and consistently detecting capture of the heart.